In passband communication systems, the carrier frequency in the transmitter may be derived locally from a timing reference, such as a crystal oscillator. A carrier recovery system may be a circuit used to estimate and compensate for frequency and phase differences between a received signal's carrier wave and the receiver's local oscillator for the purpose of coherent demodulation.
In the transmitter of a communications carrier system, a carrier wave may be modulated by a baseband signal. At the receiver, the baseband information may be extracted from the incoming modulated waveform. Ideally, the carrier frequency oscillators of the transmitter and receiver would be perfectly matched in frequency and phase thereby permitting perfect coherent demodulation of the modulated baseband signal. However, transmitters and receivers rarely share the same carrier frequency oscillator. Communications receiver systems are usually independent of transmitting systems and contain their own oscillators with frequency and phase offsets and instabilities.
In order to perform coherent demodulation, the receiver may need to know the exact carrier frequency and phase of the transmitter. Generally, the carrier frequency in the receiver may be derived independently from the transmitter. The carrier frequencies generated in the transmitter and the receiver may differ up to a maximal offset defined by a system designer. Thus the exact carrier frequency may need to be derived from the signal received at the receiver, a process also referred to as carrier recovery.
In a conventional passband communication system, the carrier recovery may be performed in at least two stages, for example, coarse frequency estimation followed by fine frequency offset estimation. It is with respect to these and other considerations that the following improvements are made.